Pigment-based black ink

ABSTRACT

A pigment-based black ink. The ink includes a carbon black pigment dispersions, macromolecular chromophores (MMCs), and an aqueous solution medium. The black ink of the present invention combines a carbon black pigment dispersions and a MMCs and has a higher optical density than ink including only carbon black pigment dispersions or MMCs

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a pigment-based black ink, andmore particularly to a pigment-based black ink containing a cabron blackpigment dispersions and a macromolecular chromophores (MMCs) pigment.

[0003] 2. Background of the Invention

[0004] Ink-jet printing technology has been developed for many years.The advantages include low price, low noise, and good full-colorprinting quality. Also, various substrates including plain paper, paperfor special printing, and transparency can be printed.

[0005] Ink-jet printing is a non-contact method that involves ejectingink droplets onto a recording substrate. For color ink-jet printing,suitable ink must meet the following requirements:

[0006] (1) Ink used has no feathering or bleeding.

[0007] (2) Ink used dries very fast.

[0008] (3) Printing nozzles do not generate clog.

[0009] (4) The ink used must have good storage stability.

[0010] (5) The ink used must be non-toxic.

[0011] Generally, few inks can meet the above requirements. For example,ink that dries very fast and has no bleeding will easily cause cloggingon the printing nozzles.

[0012] Generally, four kinds of ink are required for a color inkjetprinting machine, that is, magenta ink, cyan ink, yellow ink, and blackink. Most ink uses water-based dye. In recent years, light magenta,light cyan, light yellow, light black, orange, blue, and red inks havebeen developed for use in ink-jet printing, providing more plentifulcolor. Each can include at least one water-based dye, water, an organicsolvent and other additives. However, such water-based dye ink exhibitsvery poor waterfastness and light resistance after printing.

[0013] Therefore, many pigment-based inks have been developed in recentyears. A pigment-based ink includes at least one pigment, water, anorganic solvent, and other additives, and exhibits better waterfastnessand light resistance than a water soluble dye-based ink. A suitabledispersant and adhesive must be added to pigment-based ink. Moreover,pigment has a larger particle size, which easily precipitates andcoagulates. This causes clogging on printing nozzles and lowers theprinting quality. In order to combat this problem, it is required todecrease the coagulation force of the pigment particles by adding asurfactant such as resinous material or amine, and the solutions arecalled carbon black pigment dispersions.

[0014] U.S. Pat. No. 5,749,952 and U.S. Pat. No. 5,830,265 disclose anovel inkjet ink colorant, macromolecular chromophores (MMCs), which isa self-dispersing pigment. Such colorant is provided by Cabot and Orientcompanies. A general pigment is modified via chemical modification orion exchange process to form cationic or anionic chromophores. Takingchemical modification as an example, carboxylate functionalities (COO⁻)or sulfonate functionalities (SO₃ ⁻) are introduced onto the surface ofa general pigment to form a pigment having anionic functionalities onthe surface, called anionic chromophores. Moreover, ammoniumfunctionalities or phosphonium functionalities are introduced onto thesurface of a general pigment to form a pigment having cationicfunctionalities on the surface, called cationic chromophores.

[0015] Hewlett-Packard, in U.S. Pat. No. 5,891,934 uses a chemicalmodified MMCs and a zwitterionic surfactant to increase thewaterfastness of an ink. Hewlett-Packard, in U.S. Pat. No. 6,034,153finds that an ink containing a partially chemically modified MMCs hasbetter waterfastness than an ink containing a completely chemicallymodified MMCs pigment. Canon, in European Patent No. 1,167,470 disclosesan ink including an MMCs pigment and a benzylmethacrylate dispersant.

[0016] However, in the above conventional technology, no one has everdisclosed a way of improving the optical density of an MMCspigment-containing black ink.

SUMMARY OF THE INVENTION

[0017] One object of the present invention is to provide a pigment-basedblack ink with high optical density. The ink of the present inventionthat combines a carbon black pigment and a macromolecular chromophores(MMCs) pigment has a higher optical density than ink singly using onlycarbon black pigment dispersions or MMCs.

[0018] Another object of the present invention is to provide apigment-based black ink with excellent water resistance, and bleed andsmear resistance.

[0019] A further object of the present invention is to provide an inkjetprinting method for increasing optical density of an ink.

[0020] To achieve the above-mentioned objects, the pigment-based blackink of the present invention includes a carbon black pigment dispersionsand a macromolecular chromophores (MMCs); and an aqueous solutionmedium.

[0021] According to a preferred embodiment of the present invention, inthe pigment-based black ink of the present invention, the carbon blackpigment dispersions is present in an amount of x weight%, the MMCs ispresent in an amount of y weight%, x is between 0.01 and 10, and y isbetween 0.01 and 10, both based on the total weight of the pigment-basedblack ink. Moreover, the pigment-based black ink of the presentinvention has a higher optical density than ink containing (x+y) weight% of the carbon black pigment dispersions and containing no MMCs, andalso, than ink containing (x+y) weight % of MMCs and containing nocarbon black pigment disersions.

[0022] According to the present invention, the inkjet printing methodfor increasing optical density of an ink includes the following steps.First, a pigment-based black ink is provided. The pigment-based blackink contains a carbon black pigment dispersions and a macromolecularchromophores (MMCs); and an aqueous solution medium. Then, thepigment-based black ink is inkjet printed onto a recording substrate.

BRIEF DESCRIPTION OF THE DRAWING

[0023]FIG. 1 is a printed paper showing the black ink lines bleedinginto the yellow ink background according to the ink composition ofExamples 1 to 7 and Comparative Examples 1 to 4 of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

[0024] The pigment-based black ink of the present invention includes twokinds of pigment and an aqueous solution medium. The two kinds ofpigment are a carbon black pigment dispersions and a macromolecularchromophores (MMCs). The present invention combines these two pigmentsfor the first time and finds that ink including these two pigments has ahigher optical density than ink including only carbon black pigmentdispersions or MMCs alone. The pigment-based black ink of the presentinvention can be used in inkjet printing. In addition to high opticaldensity, the ink of the present invention exhibits good waterfastness,bleed and smear resistance, and high printing quality.

[0025] According to the present invention, the weight ratio of thecarbon black pigment dispersions to macromolecular chromophores (MMCs)can be between 1:5 and 5:1, preferably between 1:2 and 2:1. The carbonblack pigment dispersions and MMCs can have a particle size of less than1 μm, preferably 0.1 to 0.5 μm. The carbon black pigment dispersions canbe present in an amount of 0.01 to 10 weight %., and the MMCs can bepresent in an amount of 0.01 to 10 weight %, both based on the totalweight of the black ink.

[0026] According to a preferred embodiment of the present invention, inthe pigment-based black ink of the present invention, the carbon blackpigment dispersions is present in an amount of x weight %, the MMCs ispresent in an amount of y weight %, x is between 0.01 and 10, y isbetween 0.01 and 10, both based on the total weight of the pigment-basedblack ink. Moreover, the pigment-based black ink of the presentinvention has a higher optical density than ink containing (x+y) weight% of the carbon black pigment dispersions and containing no MMCs, andalso, than ink containing (x+y) weight % of MMCs and containing nocarbon black pigment dispersions.

[0027] According to the present invention, the main component of theaqueous solution medium is water. In addition to water, the aqueoussolution medium can further include an organic solvent, surfactant, pHbuffer solution, chelating agent, biocide, humectant, preservative, orUV-blocker. For example, the organic solvent can be present in an amountof 0.1 to 20 weight %, the surfactant can be present in amount of 0 to30 weight %, the humectant can be present in an amount of 0.1 to 30weight %, based on the total weight of the aqueous solution medium.

[0028] Carbon black pigment dispersions suitable for use in the presentinvention can be Bayer VPSP 20016 from Bayer, Bayer VPSP 20046 fromBayer, AcryJet series from Rohm & Haas, BASF 50087194 from BASF, BASF50007212 from BASF, Ciba B-PI from Ciba, Ciba C-WA form Ciba, SumChemical 3107 from Sum Chemical, Hostafine series from Clariant, Ilford1007-K from Ilford, MicroPigmo-series from Orient.

[0029] MMCs suitable for use in the present invention is not limited andcan comprise anionic or cationic chromophores. Anionic chromophores haveanionic functionalities such as carboxylate (COO⁻) or sulfonate (SO₃ ⁻)on the surface. Cationic chromophores have cationic functionalities suchas ammonium functionalities (NR₄ ⁺) or phosphonium functionalities (PR₄⁺) on the surface. Representative examples of MMCs include Cab-O-Jetseries (200 from Cabot, Cab-O-Jet 300) from Cabot, BonJet CW-series(CW-1 from Orient, CW-2 form Orient). Surfactants suitable for use inthe present invention can be A-102 from CYTEC, LF-4 from CYTEC, 1, 3-BGfrom KYOWA, OG from KYOWA, BEPG from KYOWA, PD-9 from KYOWA, EP-810 fromAIR PRODUCT, 1,6-hexandiol, 2,4,7,9-tetramethyl-5-decyne-4,7-diol, 1,1,1-trimethylolpropane, CT-141 from AIR PRODUCT, CT-151 from AIR PRODUCT,OT-75 from CYTEC, GPG from CYTEC, OT-70PG from CYTEC, polyethandiol,polypropandiol, EO/PO copolymer, BO/EO copolymer, sodium dioctylsulfosuccinate, alkylene oxide adduct of acetylene glycol, polybutylresin, cellulose derivative, styrene/acrylic copolymer resin, maleicacid/styrene copolymer, or a polymer containing hydrophilic segments andhydrophobic segments.

[0030] The pH buffer solution suitable for use in the present inventioncan be diethanolamine, triethanolamine, hydroxides of alkali metals suchas lithium hydroxide, sodium hydroxide and potassium hydroxide, ammoniumhydroxide, and carbonates of alkali metals such as lithium carbonate,sodium carbonate and potassium carbonate.

[0031] Chelating agents suitable for use in the present invention can besodium ethylenediaminetetraacetate, trisodium nitrilotriacetate,hydroxyethyl ethylenediamine trisodium acetate, diethylenetriaminopentasodium acetate, or uramil disodium acetate.

[0032] Organic solvents suitable for use in the present invention can becyclohexane, methanol, ethanol, 2-propanol, ethylene glycol, diethyleneglycol, triethylene glycol, propylene glycol, butylenes glycol,pentylene glycol, 2-pyrrolidone, or N-methyl-2-pyrrolidone.

[0033] The following examples are intended to illustrate the process andthe advantages of the present invention more fully without limiting itsscope, since numerous modifications and variations will be apparent tothose skilled in the art.

EXAMPLE 1

[0034] An ink composition was prepared by mixing the followingcomponents.

[0035] (1) 2 wt % of AcryJet Black 357 available from Rohm & Haas

[0036] (2) 4 wt % of BonJet CW-2 available from Orient

[0037] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0038] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0039] (5) 2 wt % of PEG 600 available from Dow

[0040] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0041] (7) 0.5 wt % GXL available from ICI

[0042] (8) deionized water

EXAMPLE 2

[0043] An ink composition was prepared by mixing the followingcomponents.

[0044] (1) 3 wt % of AcryJet Black 357 available from Rohm & Haas

[0045] (2) 3 wt % of BonJet CW-2 available from Orient

[0046] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0047] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0048] (5) 2 wt % of PEG 600 available from Dow

[0049] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0050] (7) 0.5 wt % GXL available from ICI

[0051] (8) deionized water

EXAMPLE 3

[0052] An ink composition was prepared by mixing the followingcomponents.

[0053] (1) 4 wt % of AcryJet Black 357 available from Rohm & Haas

[0054] (2) 2 wt % of BonJet CW-2 available from Orient

[0055] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0056] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0057] (5) 2 wt % of PEG 600 available from Dow

[0058] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0059] (7) 0.5 wt % GXL available from ICI

[0060] (8) deionized water

EXAMPLE 4

[0061] An ink composition was prepared by mixing the followingcomponents.

[0062] (1) 2 wt % of Bayer VPSP 20016 available from Bayer

[0063] (2) 4 wt % of Cab-O-Jet 200 available from Cabot

[0064] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0065] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0066] (5) 2 wt % of PEG 600 available from Dow

[0067] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0068] (7) 0.5 wt % GXL available from ICI

[0069] (8) deionized water

EXAMPLE 5

[0070] An ink composition was prepared by mixing the followingcomponents.

[0071] (1) 3 wt % of Bayer VPSP 20016 available from Bayer

[0072] (2) 3 wt % of Cab-O-Jet 200 available from Cabot

[0073] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0074] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0075] (5) 2 wt % of PEG 600 available from Dow

[0076] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0077] (7) 0.5 wt % GXL available from ICI

[0078] (8) deionized water

EXAMPLE 6

[0079] An ink composition was prepared by mixing the followingcomponents.

[0080] (1) 4 wt % of Bayer VPSP 20016 available from Bayer

[0081] (2) 2 wt % of Cab-O-Jet 200 available from Cabot

[0082] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0083] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0084] (5) 2 wt % of PEG 600 available from Dow

[0085] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0086] (7) 0.5 wt % GXL available from ICI

[0087] (8) deionized water

EXAMPLE 7

[0088] An ink composition was prepared by mixing the followingcomponents.

[0089] (1) 3 wt % of Bayer VPSP 20016 available from Bayer

[0090] (2) 3 wt % of BonJet CW-2 available from Orient

[0091] (3) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0092] (4) 5.8 wt % of trimethylol propane available from Aldrich

[0093] (5) 2 wt % of PEG 600 available from Dow

[0094] (6) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0095] (7) 0.5 wt % GXL available from ICI

[0096] (8) deionized water

COMPARATIVE EXAMPLE 1

[0097] An ink composition was prepared by mixing the followingcomponents.

[0098] (1) 6 wt % of AcryJet Black 357 available from Rohm & Haas

[0099] (2) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0100] (3) 5.8 wt % of trimethylol propane available from Aldrich

[0101] (4) 2 wt % of PEG 600 available from Dow

[0102] (5) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0103] (6) 0.5 wt % GXL available from ICI

[0104] (7) deionized water

COMPARATIVE EXAMPLE 2

[0105] An ink composition was prepared by mixing the followingcomponents.

[0106] (1) 6 wt % of BonJet CW-2 available from Orient

[0107] (2) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0108] (3) 5.8 wt % of trimethylol propane available from Aldrich

[0109] (4) 2 wt % of PEG 600 available from Dow

[0110] (5) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0111] (6) 0.5 wt % GXL available from ICI

[0112] (7) deionized water

COMPARATIVE EXAMPLE 3

[0113] An ink composition was prepared by mixing the followingcomponents.

[0114] (1) 6 wt % of Bayer VPSP 20016 available from Bayer

[0115] (2) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0116] (3) 5.8 wt % of trimethylol propane available from Aldrich

[0117] (4) 2 wt % of PEG 600 available from Dow

[0118] (5) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0119] (6) 0.5 wt % GXL available from ICI

[0120] (7) deionized water

COMPARATIVE EXAMPLE 4

[0121] An ink composition was prepared by mixing the followingcomponents.

[0122] (1) 6 wt % of Cab-O-Jet 200 available from Cabot

[0123] (2) 4.8 wt % of N-methyl-2-pyrrolidone available from Aldrich

[0124] (3) 5.8 wt % of trimethylol propane available from Aldrich

[0125] (4) 2 wt % of PEG 600 available from Dow

[0126] (5) 0.9 wt % of 1,6-hexanediol available from Aldrich

[0127] (6) 0.5 wt % GXL available from ICI

[0128] (7) deionized water

[0129] Printing

[0130] Each of the above ink compositions (from Examples and ComparativeExamples) was printed onto a commercially available plain paper (70 gsm)in a commercially available inkjet printer (HP DeskJet 930C).

[0131] Testing Method

[0132] (1) Optical density: The black image after inkjet printing wasexamined with a spectrophotometer (GretagMacbeth Spectroscan) foropticaldensity. The results are shown in Table 1.

[0133] (2) Waterfastness: The black image after inkjet printing wasfirst measured for optical density and then immersed in deionized waterfor 30 minutes of washing, removed, dried in the condition of roomtemperature, and measured again for optical density. ΔOD was calculated.The results are shown in Table 1.

[0134] (3) Smear resistance: A commercially available fluorescent pen(Pentel S512) was used to write on the black image after inkjetprinting. The black pigment amount carried by the fluorescent pen wasmeasured to determine the smear resistance of the black image. Theresults are shown in Table 1.

[0135] (4) Bleeding: A paper with yellow ink background was subjected toinkjet printing with each of the above black ink compositions (fromExample and Comparative Examples) in the form of lines and was thenobserved for bleeding between yellow and black. The results are shown inFIG. 1 and Table 1. TABLE 1 Carbon black pigment Mixed dispersions MMCs(Carbon black pigment Comp. Comp. Comp. Comp. dispersions + MMCs) Ex.Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. Ex. 1 3 2 4 1 2 3 4 5 6 7 Optical1.04 1.14 1.30 1.26 1.42 1.39 1.38 1.34 1.34 1.34 1.35 density Water- ⊚⊚ ◯ X ◯ ◯ ◯ Δ ◯ ⊚ ◯ fastness Smear ◯ ◯ X X Δ ◯ ◯ Δ ◯ ◯ ◯ resistanceBleeding X X ⊚ Δ ⊚ ⊚ ⊚ ◯ Δ Δ ⊚

[0136] It can be seen from the above experimental results that when anequal amount (6 weight %) of pigment is added, the ink of the presentinvention, containing both carbon black pigment dispersions and MMCs,has a higher optical density than ink containing only carbon blackpigment dispersions or MMCs. Moreover, the ink of the present inventionhas excellent waterfastness, bleed- and smear-resistance, and highprinting quality.

[0137] The foregoing description of the preferred embodiments of thisinvention has been presented for purposes of illustration anddescription. Obvious modifications or variations are possible in lightof the above teaching. The embodiments chosen and described provide anexcellent illustration of the principles of this invention and itspractical application to thereby enable those skilled in the art toutilize the invention in various embodiments and with variousmodifications as are suited to the particular use contemplated. All suchmodifications and variations are within the scope of the presentinvention as determined by the appended claims when interpreted inaccordance with the breadth to which they are fairly, legally, andequitably entitled.

What is claimed is:
 1. A pigment-based black ink, comprising: at leastone carbon black pigment dispersions and at least one macromolecularchromophores (MMCs); and an aqueous solution medium.
 2. Thepigment-based black ink as claimed in claim 1, wherein the weight ratioof the carbon black pigment dispersions to macromolecular chromophores(MMCs) is between 1:5 and 5:1.
 3. The pigment-based black ink as claimedin claim 2, wherein the weight ratio of the carbon black pigmentdispersions tomacromolecular chromophores (MMCs) is between 1:2 and 2:1.4. The pigment-based black ink as claimed in claim 1, wherein the carbonblack pigment dispersions has a particle size of less than 1 μm.
 5. Thepigment-based black ink as claimed in claim 1, wherein themacromolecular chromophores (MMCs) has a particle size of less than 1μm.
 6. The pigment-based black ink as claimed in claim 1, wherein thecarbon black pigment dispersions is present in an amount of 0.01 to 10weight %.
 7. The pigment-based black ink as claimed in claim 1, whereinthe macromolecular chromophores (MMCs) is present in an amount of 0.01to 10 weight %.
 8. The pigment-based black ink as claimed in claim 1,wherein the macromolecular chromophores (MMCS) is anionic.
 9. Thepigment-based black ink as claimed in claim 8, wherein themacromolecular chromophores (MMCs) contains carboxylate, sulfonate, or acombination thereof.
 10. The pigment-based black ink as claimed in claim1, wherein the medium further contains an additive, the additivecomprises one of an organic solvent, surfactant, pH buffer solution,chelating agent, biocide, humectant, preservative, and UV-blocker. 11.The pigment-based black ink as claimed in claim 10, wherein the mediumcontains 0.1 to 20 weight % of an organic solvent.
 12. The pigment-basedblack ink as claimed in claim 10, wherein the medium contains 0 to 30weight % of a surfactant.
 13. The pigment-based black ink as claimed inclaim 10, wherein the medium contains 0.1 to 30 weight % of a humectant.14. The pigment-based black ink as claimed in claim 1, wherein thecarbon black pigment dispersions is present in an amount of x weight %,the MMCs is present in an amount of y weight %, x is between 0.01 and10, y is between 0.01 and 10, both based on the total weight of thepigment-based black ink, wherein the pigment-based black ink has ahigher optical density than ink containing (x+y) weight % of the carbonblack pigment dispersions and containing no MMCs, and wherein thepigment-based black ink has a higher optical density than ink containing(x+y) weight % of MMCs and containing no carbon black pigmentdispersions.
 15. The pigment-based black ink as claimed in claim 14,wherein the weight ratio of the carbon black pigment dispersions to theMMCs is between 1:5 and 5:1.
 16. The pigment-based black ink as claimedin claim 15, wherein the weight ratio of the carbon black pigmentdispersions to the MMCs is between 1:2 and 2:1.
 17. An inkjet printingmethod for increasing optical density of an ink, comprising: (a)providing a pigment-based black ink, wherein the pigment-based black inkcontains at least one carbon black pigment dispersions and at least onemacromolecular chromophores (MMCs); and an aqueous solution medium; and(b) inkjet printing the pigment-based black ink onto a recordingsubstrate.
 18. The method as claimed in claim 17, wherein the weightratio of the carbon black pigment dispersions to the MMCs is between 1:5and 5:1.
 19. The method as claimed in claim 18, wherein the weight ratioof the carbon black pigment dispersions to the MMCs is between 1:2 and2:1.
 20. The method as claimed in claim 17, wherein the carbon blackpigment dispersions has a particle size of less than 1 μm.
 21. Themethod as claimed in claim 17, wherein the macromolecular chromophores(MMCs) has a particle size of less than 1 μm.
 22. The method as claimedin claim 17, wherein the carbon black pigment dispersions is present inan amount of 0.01 to 10 weight %.
 23. The method as claimed in claim 17,wherein the macromolecular chromophores (MMCs) is present in an amountof 0.01 to 10 weight %.
 24. The method as claimed in claim 17, whereinthe macromolecular chromophores (MMCs) is anionic.
 25. The method asclaimed in claim 24, wherein the macromolecular chromophores (MMCs)contains carboxylate, sulfonate, or a combination thereof.
 26. Themethod as claimed in claim 17, wherein the carbon black pigmentdispersions is present in an amount of x weight %, the macromolecularchromophores (MMCs) is present in an amount of y weight %, x is between0.01 and 10, y is between 0.01 and 10, both based on the total weight ofthe pigment-based black ink, wherein the pigment-based black ink has ahigher optical density than ink containing (x+y) weight % of the carbonblack pigment dispersions and containing no MMCs, and wherein thepigment-based black ink has a higher optical density than ink containing(x+y) weight % of MMCs and containing no carbon black pigmentdispersions.
 27. The method as claimed in claim 26, wherein the weightratio of the carbon black pigment dispersions to the MMCs is between 1:5and 5:1.
 28. The method as claimed in claim 27, wherein the weight ratioof the carbon black pigment dispersions to the MMCs is between 1:2 and2:1.